Oujisama to Haiiro no Hibi

A drunkard for a father, a household of poverty... For 17-year-old Atsuko, misfortune is all she knows and believes in. Until one day, a chance encounter with Itaru—the wealthy heir of a huge corporation—changes everything. The two look identical, uncannily so. When Itaru curiously goes missing, Atsuko is roped into being his stand-in. There, in his shoes, Atsuko must parade like a prince in a palace. She encounters many new experiences, but at what cost...?
(Source: Kodansha USA)